Yeah, it's one of those words that has some cultural meaning attached to it. If you aren't a U.S. or British native speaker then you might not "get it" and the dictionary doesn't really explain. If you ever need to know the meaning of a slang term or a dictionary definition doesn't seem correct in the way the term was used, then check the Urban Dictionary. Trifecta basically means a perfect three of something. Urban Dictionary: trifecta